Methods for preventing corrosion of a metal surface include a zinc or zinc alloy plating method and the like. The corrosion resistance achieved by the plating alone is insufficient. Hence, a post-plating treatment with chromic acid containing hexavalent chromium, so called the chromate treatment, is widely employed in the industrial field. However, it has been pointed out recently that hexavalent chromium has adverse effects on the human body and the environment, and the use of hexavalent chromium has been restricted. An alternative technology is a corrosion inhibition coating film using trivalent chromium. For example, Japanese Patent Application Publications Nos. 2000-509434 and 2004-003019 disclose a treatment method using a processing solution comprising a trivalent chromium salt, nitrate ions, an organic acid, and a salt of a metal such as cobalt. Meanwhile, Japanese Patent Application Publications Nos. 2001-335958 and 2005-126797 disclose a treatment method using a trivalent chromium chemical conversion processing agent for zinc alloy plating, the agent comprising a trivalent chromium salt, nitrate ions, an organic acid, and a salt of a metal such as cobalt.